Re: [RFC PATCH v1 1/2] x86/mm: use VMA lock for kernel faults on user addresses

On Sun, Aug 02, 2026 at 03:40:17PM +0800, Barry Song (Xiaomi) wrote:
> Use the VMA lock for kernel faults on user addresses. This also
> makes the existing code below meaningful:
>
>   /* Quick path to respond to signals */
>   if (fault_signal_pending(fault, regs)) {
>           if (!user_mode(regs))
>                   kernelmode_fixup_or_oops(regs, error_code, address,
>                                            SIGBUS, BUS_ADRERR,
>                                            ARCH_DEFAULT_PKEY);
>           return;
>   }

Hmm yeah :)

Bit weird it uses user_mode(regs) and the early exit uses the just-set
'flags & FAULT_FLAG_USER' too.

Some horrible duplication here too... the user_mod_regs() etc. code is just
duplicated in the mmap lock path.

I think you mentioned it on Suren/Dave's series but vma_start_read_unlocked()
would avoid all this and could lead to a nicely red patch.

>
> Right now, the code above is dead because !user_mode always
> takes the mmap_lock path.
>
> Co-developed-by: Bo Zhang <[email protected]>
> Signed-off-by: Bo Zhang <[email protected]>
> Signed-off-by: Barry Song (Xiaomi) <[email protected]>
> ---
>  arch/x86/mm/fault.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 45b99c3b1442..c22b74e0eeaf 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1328,9 +1328,6 @@ void do_user_addr_fault(struct pt_regs *regs,
>  	}
>  #endif
>
> -	if (!(flags & FAULT_FLAG_USER))
> -		goto lock_mmap;
> -

I'm not sure why kernel faults of userland memory were excluded initially
(Suren?)

Given that it's OK to take the mmap lock here and we're necessarily in process
context anyway surely it's OK to take the VMA lock?

It looks fine to me but want Suren's input.

Also given vma_start_read_unlocked() is coming maybe that's better for a
cleanup.

And finally - Willy is working on a grand clean up of this stuff _I think_ so
you probably should coordinate with him also?

>  	vma = lock_vma_under_rcu(mm, address);
>  	if (!vma)
>  		goto lock_mmap;
> --
> 2.39.3 (Apple Git-146)
>

--
Cheers, Lorenzo